Mercury vapor electron generator



Sept. 8, 1970 MEAD 3,527,975

MERCURY VAPOR ELECTRON GENERATOR Filed Oct. 11, 1967 2 Sheets-Sheet 1 15 80 \i o o 0 4 x88 o o o 0000000 o o o o z: z; 94 43 2\ l2 o oo 42 oo o %o I 1 9 000 o 0O \i 39 0C 1 86J 38 ,1 76 28 t g F 76 \Y 30 32 I 3 /T} 26 Q 40 -4e 34 I 5 2 J8 O 48 m INVENTOR.

V GEORGE N. J. MEAD BY 777m lZZM/ ATTORNEYS G. N. J. MEAD MERCURY VAPOR ELECTRON GENERATOR Sept. 8, 1970 r 2 SheeTs-Shee1 u Flled Oct. 11. 1967 mvEN'rm GEORGE N. J. MEAD BY ATTORNEYS FIG. 6

United States Patent US. Cl. 313-171 20 Claims ABSTRACT OF THE DISCLOSURE A mercury cathode device that is compatible with evacuated systems is provided for various electrical and electronic applications and is characterized in operation by high power density at a low voltage drop. Mercury flows over a conical electrode where it forms a thin film which is the emission surface and which is continuously regenerated by the recirculating mercury. The thin film provides stability for the cathode spot with anchored emission and also prevents the boiling and turbulence normally associated with mercury pool cathodes. A baflle is located above the electrode and the assembly is surrounded by an annular positively charged electrode which attracts the electrons radially outward. As the emitted electrons approach the periphery of the battle they are caught in a cylindrical annular magnetic field at the periphery of the emission surface. The magnetic field directs the electrons into a circular path preventing them from reaching the peripheral electrode. The rotary movement of the electrons in this area at high speeds ionizes any mercury atoms that may have escaped from the emission surface or elsewhere. Positive ions produced by the ionization are then attracted back to the emission surface rather than advancing further into the system. The ionization zone acts as a barrier to the passage of all particles except electrons thus maintaining a plasma in close proximity to the cathode and making the unit compatible with evacuated systems.

Located above the baffle and radially inward from the peripheral electrode is an annular passage in which is located an electron extracting device in the form of an open grid work electrode having a greater positive charge than that on the peripheral electrode. Thus the orbiting electrons are extracted and directed inward to form a converging electron beam.

BACKGROUND OF THE INVENTION Field of the invention This invention relates generally to mercury cathode devices and more particularly is directed towards a new and improved mercury cathode providing stable emission at both high and low current densities and capable of operation with a vacuum system.

Description of the prior art While vacuum type electronic devices are used in a Wide variety of applications, their use is still somewhat limited by the fact that they are incapable of operating at high flower densities and low voltage drops. Conventionally vacuum electronic devices employing thermionic cathodes normally operate with a maximum current density of about 3 amps/cm. Mercury pool cathodes operate on current densities on the order of 10 amps/cm. at the cathode spot and at low voltage drops. In order for mercury pool cathodes to be used in conjunction with vacuum electronic devices the mercury atoms and ions must be retained in the immediate vicinity of the cathode and any mercury atoms that escape should be returned to the cathode. Various techniques have been proposed in an attempt to control the emission from the mercury in a controlled manner. Such devices heretofore have not been entirely satisfactory for various reasons. It is, therefore,

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an object of the present invention not only to improve mercury cathodes generally but to provide a very etficient mercury cathode and one which does not display boiling and explosive ejection of mercury droplets during operation. A further object of the invention is to provide a mercury cathode in which the emission is anchored and stable at both high and low current densities. Another object of this invention is to provide a mercury type of cathode that is compatible with vacuum systems.

The device is useful for applications such as rectifiers or inverter valves, microwave amplifier tubes, X-ray tubes and electron beam welding. The device may also be used as a source of electrons in a low loss electric power conductor of the sort disclosed in my co-pending application Ser. No. 360,277 now US. Pat No. 3,364,389.

SUMMARY OF THE INVENTION The present invention features a mercury cathode having a negative electrode, preferably in the form of a cone and over the surface of which is pumped continuously a cooled film of mercury which provides the emission surface. A catch basin is provided to recover the mercury as it runs off the electrode and a pump operates continuously to provide a flow up to the apex of the conical electrode. The mercury film provides a very stable emission surface not subjected to boiling or explosive ejections and which may be maintained at cooler operating temperature than conventional mercury cathodes. A battle is located above the conical electrode and a peripheral electrode draws the electrons from the mercury film outwardly where they are trapped by a magnetic field which causes complete ionization of the mercury vapor. Positive mercury ions are returned to the cathode while the electrons are drawn off by an extraction electrode located above the baffie across an annular passage whereby the electrons are directed towards a focused beam.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view in side elevation of a mercury cathode device made according to the invention,

FIG. 2 is a detailed crosssectional bottom plan view of the pump employed for the device in FIG. 1,

FIG. 3 is a cross-sectional view taken along the line 33 of FIG. 1.

FIG. 4 is a fragmentary sectional side view of one modification of the invention,

FIG. 5 is a fragmentary cross-sectional View in side elevation showing another modification of the device, and,

FIG. 6 is a schematic diagram of the FIG. 1 device.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings the mercury cathode is generally organized within a housing 10 which is spaced from an annular cup 12 to form a cooling jacket in the surrounding space 14 through which a suitable cooling medium is pumped in order to maintain the cathode at a lower operating temperature. The cup 12 carries upper and lower magnetic poles 16 and 18 respectively, the opposing faces of which are spaced from one another and define with the cup 12 an annular volume 20. Disposed in the upper portion of the volume 20' between the upper pole 16 and the cup 12 are electro-magnetic coil windings 22 for reasons that will presently appear The lower portion of the volume 20 serves as the catch basin for liquid mercury 24 with the catch basin being separated from the windings 22 by means of an annular shield 26 which is charged negatively to the same potential as the mercury and which is mounted to the cup 12. Mounted inwardly of the shield 26 is an annular dielectric layer 28 which extends from the lower end of the upper pole 16 to a point just above the upper surface of a generally conical electrode 30 mounted centrally of the unit. The

lower end of the insulating layer 28 extends inwardly over the edge of the electrode 30 to form an annular shield 32 defining a narrow annular opening 34 to the catch basin. Mounted on the inner face of the insulating layer 28 is an arcuate annular electrode 36 which hence forth will be termed the peripheral electrode.

The conical electrode 30 is formed with internal cooling passages 38 and a central opening 40 up through which mercury is continuously pumped and is peripherally supported on the upper end of the lower magnetic pole 18. Mercury delivered up through the opening 40 will strike an overhead baffle 42 to be directed down- ,wardly onto the upper surface of the electrode 30 where it will form a thin film flowing down over the entire conical surface through the opening 34 and into the catch basin 20. The mercury is delivered up through the opening 40 by means of a pump, generally indicated by reference character 44 which pumps the mercury up through a central vertical tube 46. The pump is located below a spring-loaded piston 48 mounted for reciprocation within a cylinder 50 whereby a relatively large amount of mercury may be stored and yet the system will be capable of operation with a small amount.

The piston and cylinder provide an internal variable capacity reservoir Within the electrode with the springloaded piston separating the top from the bottom of the reservoir. The piston is guided by the tube 46 that extends from the pressure discharge side of the pump. Mercury in the upper portion of the cylinder is under pressure maintained by the pump while the mercury in the lower portion is connected to the catch basin through drains 52 and to the suction side of the pump. In practice, the outer edge of the piston is sealed by means of a bellows 54 which may also provide the spring-loading. Owing to the small circumference where the center of the piston slides on the vertical tube the leakage area is held to a minimum. The clearance volume on the piston at the top of its travel is close to zero and this arrangement permits storing a large amount of mercury and yet is capable of operation with a small amount. This feature, therefore, may be used to make up for mercury losses and to minimize sloshing of mercury under the influence of external accelerations.

The pump 44, in the preferred mode, comprises a movable vane rotary pump integrated into the rotor of a homopolar motor. The configuration is such that both the rotor of the pump and the motor are one and the same. As shown in FIGS. 1 and 2 the disc-shaped rotor is indicated by reference character 56 within a housing 58. The rotor thickness is small in relation to the diameter and the rotor is covered with a layer of dielectric insulating material except for the outer periphery and the bottom of its hub 59. The mercury, which is negatively charged, surrounds the entire rotor except for the bottom of the hub where the mercury is sealed off. The bottom of the hub is connected to the positive terminal of a battery 60 through a thrust bearing 62. This electrical contact area may be extended by using a drop of liquid mercury in the thrust bearing and this will also lubricate the bearing. In this fashion, electrical current flows through the hub and radially outward of the rotor disc. The mag netic field flux is directly axially across the rotor thicknes and is produced by permanent magnet end plates 64 and 66. Radially movable vanes 68 and 70 are springloaded in radial slots 72 formed in the rotor whereby the outer edges of the vanes bear against the inner surface of the cylindrical housing 58. The base of each vane slot is vented to the vane tip by means of a passage drilled lengthwise through the vane.

The suction port of the pump is located in the bottom end plate while the pressure port is located in the top end plate and comunicates with the vertical tube 46 as best shown in FIG. 1.

The thrust bearing for the pump and the positive electrical contact in the bottom of the hub rotor are suitably 'sealed from the negatively charged mercury as by means of labyrinth seals, by the use of materials not wettable by mercury, or by a simple dynamic slinger to eject nega tively charged mercury from the contact space.

In practice, the power supply for the motor can be the same as that used for applying a positive potential to the peripheral electrode 36. The power requirement for the pump is low and homopolar motors have no eddy current or hysteresis losses since there are no flux reversals. Since the pump is of the negative displacement type losses are minimal and the low operating speed produces very little wear. Low losses have the further abvantage of not adding significant heat to the mercury.

The conical electrode 30, as previously indicated, is formed with cooling passages 38 which communicate with the cooling jacket 14 through a passage 74. The upper surface of the electrode 30 is somewhat curved in crosssection from the apex to the periphery for reasons that will be presently seen. The top surface of the electrode 30 preferably is coated with a layer of molybdenum to protect the electrode surface from deterioration and it is over this molybdenum coated surface that the film of mercury forms and flows. The mercury film provides the actual emission surface and is continuously regenerated by the pump 44 which re-circulates the mercury in the manner previously described. The conical electrode 30 is, of course, connected to the negative terminal of the power supply. In practice, the upper surface of the conical electrode may be formed with shallow spiral grooves 76 to insure that the mercury covers the entire emission surface as it flows down from the apex of the cone. The spiral grooves will lead the mercury around the cone surface and will also slow down its flow. In addition, the spiral grooves create a roughness in the emission surface and thus increase the local electrical field gradient. In practice, the conical electrode 30 is made of a highly conductive material which, combined with its water cooling system, permits it to handle a high current density with low losses.

The use of the thin film of mercury as the emission surface presents several distinct advantages. First of all, it provides over a wide area the same thin film that pertains in anchored emission along the thin line where the mercury meniscus wets a molybdenum anchor. This thin film is basic to the stability of the cathode spot with anchored emission. Secondly, the thin film prevents the violent turbulence that results in explosive boiling common in conventional mercury pool cathode devices. Thirdly the thin film is easily cooled so as to prevent excessive evaporation and, fourthly, the thin film reduces the resistance of electrical current flow through the mercury. It is, of course, desirable that the mercury or the molybdenum surface be treated with sodium or zirconium, for example, so that the mercury will wet the electrode to minimize electrical resistance and maximize heat transfer. Furthermore, the use of the thin, cool, mercury film permits operating with a small quantity of mercury while providing a large emission surface area.

The shield 32 which surrounds the peripheral margin of the electrode 30 also covers the catch basin to deter emission from the basin and also prevents the liquid mercury from spilling out of the basin if the cathode device is subjected to external accelerations.

The ignition of the arc is provided by a static ignitor 80 which extends vertically down through the baffle 42 with the lower tip of the ignitor projecting into the mercury discharge opening 40 of the conical electrode 30. The ignitor 80, in practice, is fabricated from a refractory material of high resistivity such as boron carbide or silicon carbide. The ignitor should not be wet by the mercury; therefore wetting agents added to the mercury for wetting the molybdenum surface of the conical electrode should not also cause wetting of the ignitor. By locating the tip of the ignitor immersed in the discharge opening at the apex of the conical electrode, the ignitor is centrally located and connects to the entire emission surface and yet does not interfere with the magnetron type of ionizing action to be described below.

If the wetting agents in the mercury impair the performance of the static ignitor described above a different type of ignitor may be provided. In the modification illustrated in FIG. 4, an ignitor electrode 82 is mounted to a piezoelectric transducer 84 with the tip of the ignitor extending to the mercury surface. By driving the transducer at a low ultrasonic frequency cavitations will be produced in the mercury where the ignitor touches the mercury surface. This, in efiect, produces many make and break contacts between the ignitor and the mercury and will promote fast, reliable starting of the arc. The transducer vibration also serves as a mechanical atomizer to create a mist of neutral mercury particles above the emission surface. This mist will aid in the formation of a mercury plasma and arc discharge.

The baffle 42 is of circular outline slightly larger in diameter than the conical electrode 30 and is placed above the electrode 30. The baflle 42, in practice, is from a dielectric material or otherwise electrically insulated from the system. The upper face of the battle 42 is formed with a curving slope to define a smoothly, curving annular passage 43 from its periphery to its central axis. Also the baffle 42 is formed with internal passages 86 for the cooling medium, these passages being connected to the water jacket 14 by means of a passage 88 which connects with the bafile passage 86 by means of tubes 90' extending across the annular passage 43 between the lower leg of the upper pole piece 16 and the upper peripheral surface of the bafiie 42.

Mounted in an annular array across this passage and generally concentric with the tubes 90 is an extraction electrode 94 typically in the form of an openwork grid. The extraction electrode is located radially inward from the peripheral electrode 36 and defines an annular opening leading back towards the the axial center line of the system. The extraction electrode carries an equal or greater positive charge than the charge on the peripheral electrode. In association with the extraction electrode the tubes 90 may be located in close proximity to one another to define insulated constrictions which will serve to inhibit passage of positive ions as will appear more fully in the description of the operation of the device to follow.

Most low pressure mercury arc devices operate at approximately 4% ionization. However, for a mercury cathode to be compatible with vacuum electronic devices substantially 100% ionization is required. Thus every neutral mercury particle must be ionized so that the positive ions will be positively attracted back to the negatively charged emission surface. Ionization is best accomplished by means of electron bombardment in a magnetron type system. The high ionization also improves emission as well as enhancing current density.

In the operation of the cathode described above when the coil 22 and electrodes are energized a high local electrict gradient is applied to the emission surface namely, the mercury film on the conical electrode 30, and the peripheral electrode attracts all emitted electrons radially outwards. The electrons emitted from the mercury film are prevented from reaching the peripheral electrode because of the annular magnetic field produced by the coil 22. This magnetic field is formed in the annular clearance between the peripheral electrode and the outer edge of the bafile 42. When the radially moving electrons impinge upon this magnetic field, they are caught up and bent into a circular path by the magnetic field and cannot reach the peripheral electrode. Under these conditions all of the emitted electrons sweep the entire periphery of the emission space at high speeds and any mercury atoms that escape from the emission surface or from the catch basin will be ionized. This type of ionization is equally effective at partial loads as at a whole load and has the further advantage that the energy required per ion pair is relatively low. The magnetic field is known as the Brillouin type and forces the electrons into orbital paths around the emission surface rather than returning them to the center line of the system. The configuration is such that the magnetic field need fill only a small volume and if sufficiently intense can contain the mercury plasma.

The shield 32 located above the face of the lower magnetic pole is electrically insulated and prevents emission from the pole as well as preventing return of positive ions to that surface while at the same time permitting the mercury to flow under the shield to the catch basin.

In the ionization process positive ions are produced and these are attracted back to the emission surface on the conical electrode rather than being carried into the vacuum electronic device. The ionization zone thus acts as a barrier to retain the plasma in close proximity to the cathode and preventing it from entering a high vacuum volume elsewhere in a system. As the positive ions return to the emission surface, they perform two important functions, namely, they provide, first of all, an extremely steep localized electrical field gradient immediately above the emission surface and, secondly, they bombard the emission surface and cause highly localized surface boiling which in turn provides the neutral particles that are ionized to produce the mercury plasma in contact with the emission surface.

The electron extraction electrode 94 located across the passage 43 may be covered by electrical insulation and carries a greater positive field gradient owing to its smaller radius of curvature than that on the peripheral electrode whereby the orbiting electrons which are able to move parallel to the magnetic lines of force are attracted around the outer edge of the baffle 42 and proceed through the extraction electrode. This produces a converging electron beam which can then be further shaped by electronic lenses or the like located in a passage 96 above the baffle 42.

Where the cathode is used in a rectifier system having non-conducting periods, a keep-alive circuit may be added to maintain the arc. This function may be carried out by the extraction electrode the surfaces of which would not be insulated and in addition placing a high resistance in the wire connecting the extraction electrode to the battery terminal so as to limit the current flow.

In the event that a second stage of ionization is desirable, a constrictor plate formed with a series of slots or apertures may be employed across the passage 43 to increase the current density and to ionize any neutral particles that might escape the magnetron type of ionization.

By shaping the conical electrode 30 with its upper surface in the form of a curved slope a more evenly distributed emission will take place insofar as the apex of the conical electrode which is furthest from the peripheral electrode will present a surface which will be more nearly parallel the peripheral electrode than the lower outer edge of the conical electrode where the surface is more at an angle to the peripheral electrode. Also, because of the smaller radius of curvature near the apex a higher charge or gradient will be present at the apex.

The mercury atoms that may happen to escape the cathode will most likely be ionized by reason of the high current density electron flow. When this happens there exists a favorable electrical field gradient back to the emission surface so that these positive ions will be returned to the cathode. Furthermore, if the electron velocities are in the proper range they will also ionize any gas atoms or molecules that might leak into the system.

When mercury ions are returned to the cathode, the liquid mercury serves as a getter. However, when the gas ions are returned by ionic pumping, as described above, they must be removed. Accordingly, for a system with a large surface area and a good chance of leakage, a vacuum booster pump must be provided with a connection 7 which can be made adjacent the peripheral electrode, for example.

The present invention is compatible with vacuum electronic devices insofar as the mercury is retained in the cathode. This is accomplished by reason of the fact that the mercury is cooled throughout its circulation path so that evaporation is held to a minimum. Secondly, the use of a thin, cooled film for the emission surface prevents violent boiling with forcible ejection of liquid mercury drops as is common with conventional mercury pool cathodes. Thirdly, the ionizing system features high ionization elficiency and makes possible positive retrieval of mercury atoms by means of ionic pumping.

The configuration of the mercury cathode enhances the emission stability. The requirements of stable emission are a high electric field gradient, sufiicient mercury vapor pressure and sufiicient positive ion bombardment to produce the mercury vapor. The present cathode meets all of these requirements. It will be noted that although the amount of mercury vapor and the vapor pressure are reduced, the percentage of ionization is greatly increased. Accordingly, the positive ions that create the steep voltage gradient as they return to the emission surface will be increased with a corresponding increase in emission. In addition the positive ion bombardment is correspondingly increased by the increased ionization so that enough surface boiling and small vapor particles will be produced. The thickness of the mercury film on the conical electrode surface plus the cooling rate will be adequate to avoid evaporating the film dry under the cathode spots.

Since the plasma is confined to the immediate vicinity of the emission surface, the problem of emission instability caused by the arc shunting through a localized highly ionized space with concurrent reduction in the arc voltage drop, is avoided.

Insofar as electrons are much lighter and more agile than positive ions they are easily controlled by a magnetic field. Positive ions, on the other hand, are best controlled by electrostatic fields. Thus when the electrons meet the magnetic field they will orbit the emission surface while the positive ions will return to the emission surface before they begin to orbit. This will produce an orbiting cloud of electrons outside of the mercury plasma. The volume for accumulating space charges is kept small so that space charge effects will not dominate the operation of the cathode.

Referring now more particularly to FIG. there is disclosed a modification of the invention and in this embodiment extraction of the electrons from the field of the electro-magnetic winding is carried out by means of a low-loss conductor generally indicated by reference character 100. The low-loss conductor is mounted on top of a baflle 42' in a passage 43 and comprises an annular body formed with one or more insulated capillary passages 102 which are encapsulated in an interconnected conducting metal matrix 104. The matrix 104 is provided with a positive electrical charge whereby the space charge of the electron stream flowing through it will be neutral ized and will not obstruct current fiow. By increasing the positive static charge on the matrix the passages will be able to carry a high current density and the electrons will follow the passages even if they are curved as in the FIG. 5 embodiment.

The low-loss conductor mentioned above is more fully disclosed in applicants co-pending patent application Ser. No. 360,277, filed Apr. 16, 1964.

By providing a cooling jacket 106 on the upper curved surface of the low-loss conductor and a similar curving passage 108 in the bafiie 42' the conductor also serves 4 the passages and will be condensed to a liquid. The passages are oriented .so that condensed mercury will run down and return to the emission surface. The metal matrix may be chilled with liquid nitrogen or other means of refrigeration. The low-loss conductor may be used conveniently as a transition duct to extract electrons from the zone near the peripheral electrode and carry them on a converging path to form an electron beam parallel to the axial center line.

Having thus described the invention what I claim and desire to obtain by Letters Patents of the United States is:

1. A mercury cathode device, comprising (a) an enclosure,

(b) at least a pair of electrodes mounted in spaced relation to one another within said enclosure,

(c) a pump adapted to discharge a flow of mercury over one of said electrodes,

(d) first means for energizing said electrodes whereby electrons will be emitted from the mercury flowing over said one electrode, and,

(e) second means for preventing said mercury from contacting the other electrode.

2. A mercury cathode according to claim 1 wherein said one electrode is formed with an inclined surface over which said mercury flows.

3. A mercury cathode according to claim 1 including a baffle disposed between said pair of electrodes, a third electrode disposed about said one electrode and bafiie in spaced relation thereto for attracting emitted electrons towards the edge of said baflie and means forming a magnetic field about said bafiie for temporarily trapping emitted particles and causing ionization thereof.

4. A mercury cathode according to claim 3 including a fourth electrode disposed between the other of said pair of electrodes and said field for extracting electrons therefrom.

5. A mercury cathode according to claim 1 wherein said one electrode is formed with a conical surface.

6. A mercury cathode according to claim 1 wherein said pump includes a homopolar motor.

7. A mercury cathode according to claim 1 including a cylinder communicating with said pump and adapted to store a quantity of mercury and a spring-loaded piston mounted in said cylinder to form a variable volume mercury reservoir.

8. A mercury cathode according to claim 1 including an ignitor within said enclosure and extending to the surface of said mercury and means for oscillating said ignitor to and away from said mercury.

9. A mercury cathode, comprising (a) an enclosure,

(b) a first electrode having a conical upper surface mounted upright in said enclosure;

(0) pumping means for delivering a flow of mercury to the apex of said conical surface whereby said mercury will flow downwardly and outwardly in a conical film over said surface,

(d) conduit means connected to said pump for recovering and recirculating said mercury,

(e) a bafiie mounted above said first electrode and generally coextensive therewith,

(f) a second electrode mounted in spaced relation about said first electrode and adapted when energized to attract emitted particles from said mercury film radially outwards towards the periphery of said baflie,

(g) means for forming a magnetic field between said firs; and second electrodes to ionize said particles an (h) means for extracting electrons from said field and direct them above said bafile.

10. A mercury cathode according to claim 9 wherein said extracting means is a third eletcrode disposed inwardly of said second electrode and above said baflie.

11. A mercury cathode according to claim 9 wherein said extracting means includes a low loss conductor disposed on the upper surface of said bafile and having its lower end in the vicinity of said field and its upper end above said bafiie, said conductor having a conductive matrix formed with at least one insulated passag eand means for placing charge on said matrix.

12. A mercury cathode according to claim 9 including a variable volume reservoir for said mercury in communication with said pump.

13. A mercury cathode according to claim 12 wherein said reservoir includes a cylinder to receive said mercury and a spring-loaded piston mounted within said cylinder.

14. A mercury cathode according to claim 9 wherein the surface of said first electrode is formed with a plurality of generally circular spaced concentric grooves.

15. A mercury cathode according to claim 9 including an ignitor extending towards the apex of said first electrode.

16. A mercury cathode according to claim 15 including means for oscillating said ignitor into and out of contact with said mercury.

17. A mercury cathode according to claim 9 wherein said field forming means includes magnetic pole pieces and windings disposed about said first and second electrodes.

10 out of contact with said surface.

References Cited UNITED STATES PATENTS 15 1,848,279 3/1932 Spagnola 313-12 X 1,873,963 8/1932 Jonas 313173 X 1,904,396 4/1933 Widmer 313-173 X 2,225,757 12/1940 Ramsay 313328 X 20 JAMES W. LAWRENCE, Primary Examiner C. R. CAMPBELL, Assistant Examiner US. Cl. X.R. 

